Hi Reino,
Thanks for responding.  To answer your question... Yes, I can paste into Notepad just fine.
I think I see the problem.  Perhaps you can test this to see if you get the same behavior.
If I select a call sign with an extra space such as "3D2ZK ", copy it to the clipboard, and then try to paste it into WSJT-X, it fails to paste.
If instead I select a call sign with no extra spaces such as "3D2ZK", it seems to work.
Perhaps the clipboard handler in WSJT-X could be more forgiving by trimming whitespace off of the text rather than rejecting it completely.
AK2L, David